Phased array antennas are well known in the art provide for wide bandwidth, low power losses, and have the capability of radiating at multiple frequencies in a given beam direction. The use of the invention is thus not intended to be restricted to any particular type of aircraft. Thus regardless of the size or type of aircraft, some kind of radar system is required. Long range radar with 360°-coverage requires a large antenna mounted above the aircraft fuselage, to minimize airframe interference, and requires high power to give the required long-range detection. These two requirements have so far been to a certain extent mutually incompatible since a large antenna mounted above the aircraft fuselage causes substantial drag and coupled with the high power requirements of the long-range radar only a large aircraft could be used.
U.S. Pat. No. 5,097,267 (Raviv) published Mar. 17, 1992, assigned to the present applicant and entitled “Airborne early warning radar system” discloses an airborne early warning radar system comprising an automatically controlled pilotless aircraft, a phased array radar antenna disposed in the pilotless aircraft and apparatus for selectably varying the orientation of the phased array radar antenna relative to the aircraft.
All known AEW (Airborne Early Warning) systems, including AWACS, ERIEYE, CONDOR and WEDGETAIL have been based on the traditional architecture of a single-band radar. AWACS employs a mechanically rotating antenna, while the other systems employ an electronically scanned fixed array antenna of the kind described in above-mentioned U.S. Pat. No. 5,097,267.
In the mechanically rotating system, the time required to achieve a full 360° scan is dependent on the time taken to steer the mechanical antenna through a complete 360° revolution. Although the steering of a phased array radar antenna is electronic, a complete electronic scan is nevertheless required to achieve full 360° coverage. Once the radar has tracked a point in space, the same point will not be “visible” to the radar during the current cycle. This means that the longer it takes for a complete scan, the easier it is for a foe to penetrate the radar, since it has more time to intercept and having done so, the more time will elapse until it becomes visible to the radar during a subsequent scan. Consequently, it is desirable to reduce the time for a complete 360° scan.
U.S. Pat. No. 4,870,426 (Lamberty et. al.) published Sep. 26, 1989 and entitled “Dual band antenna element” discloses a radar antenna element comprising a lower band waveguide and an array of parallel, dual-polarized, higher band waveguides and dipoles mounted within or directly adjacent an aperture of the lower band waveguide. The lower band waveguide and each higher band waveguide have one cross-sectional dimension less than 0.5 wavelength.
U.S. Pat. No. 4,743,907 (Gellekink) published May 10, 1988 and entitled “Radar system operating in two frequency bands” discloses a radar system for low-level target tracking comprises a first and a second radar apparatus, both operating at their own frequency, but employing one and the same tracking antenna.
U.S. Pat. No. 4,276,551 (Williams et. al.) published Jun. 30, 1981 and entitled “Electronically scanned antenna” discloses electronically scanned antennas comprising a plurality of frequency scanned antenna sections each of which have a plurality of radiating elements and a plurality of phase shifters individually coupled to the antenna sections. Such an arrangement uses dual frequency for steering the radar beam but does not employ different scan frequencies.
There is no suggestion in the above references to employ a phased array antenna comprising at least two cyclically selectable antenna couplets each comprising a pair of adjacent antennas adapted for simultaneous operation at different frequencies.
It is an object of the present to provide a more compact phased array radar antenna that permits a complete 360° coverage to be achieved.